Process for reducing discoloration of electrochemically treated chromium plated ferrous metal strip

ABSTRACT

Discoloration of ferrous metal strip electroplated with metallic chromium in a continuous electroplating line at high line speeds and then electrochemically treated to deposit a nonmetallic chromium containing film thereover is reduced by intimately contacting the metallic chromium plated strip with an aqueous solution of a water soluble hexavalent chromium compound prior to the electrochemical treatment. The aqueous solution preferably has a composition which is substantially the same as the chromium electroplating electrolyte. The invention is especially useful in producing tin free ferrous metal container stock having a composite protective coating including an undercoating of 0.1-0.5 microinch of metallic chromium and an overcoating containing chromium oxide.

United States Patent Austin PROCESS FOR REDUCING STRIP 75 Inventor:Lowell w. Austin, Weirton, w. Va.

[73] Assignee: National Steel Corporatio Pittsburgh, Pa. I

' 221 Filed: June 19,1969

[2]] Appl. No.: 834,872

[52] 1.1.8. Cl..... 204/35 N, 204/35 R, 204/51 [51] Int. Cl...... C32b11/00, C23b 5/48, C23b 5/06 [58] Field of Search 204/35 R, 36, 41,204/35, 51

[56] v References Cited UNITED STATES PATENTS 3,642,587 2/1972 Allen eta1. 204/35 N 2,438,013 3/1948 Tanner 204/35 X 2,746,915 5/1956 Giesker204/56 3,113,845 12/1963 Uchida et al. 204/41 X 3,216,912 11/1965Shoemaker 204/35 X 3,296,100 1/1967 Yonezaki et al 204/41 3,323,8816/1967 Nelson et a1 29/1835 3,498,892 3/1970 Seyb, Jr. et al. 204/51 X3,526,486 9/1970 Smith et al 204/41 3,316,160 /1967 Uchida et a1. 204/283,567,599 /l 971 Carter et a1 204/41 Belgium [11] 3,755,091 1 Aug. 28,1973 OTHER PUBLICATIONS Smith, E; .l., Chromium Coated Steel ForContainer Applications, prepared for presentation at 75th GeneralMeetingof Amer. Iron and Steel Institute, in New York, May 25, 1967 Graham, A.K. Electroplating Engineering Handbook (1st ed.) 1955, Reinhold Pub.Corp. Baltimore, Md,

.1. of Electrochem. Soc., March 1969, Fukuda et a1. (presented Oct.1967) Y Priniary Examiner-F. C. Emundson Attorneyl-Shanley and ONeil 57ABSTRACT solution preferably has a composition which is substantiallythe same as the chromium electroplating electrolyte. The invention isespecially useful in producing tin free ferrous metal container stockhaving a composite protective coating including an undercoating of0.1-0.5 microinch of metallic chromium and an overcoating containingchromium oxide.

11 Claims, N0 Drawings BACKGROUND OF THE INVENTION This inventionbroadly relates to a process for reducing discoloration of ferrous metalstrip electroplated with metallic chromium in' a continuous high speedchromium electroplating line. In one of its more specific variants,theinvention is concerned with the preparation of thin gauge tin freeferrous metal strip useful as'a container stock.

Tinplate is widely used in manufacturing containers for storing andpreserving foods and beverages. However, in view of the high cost of tinand the lack of a dependable source of tin in this country, it isdesirable to provide tin free corrosion resistant ferrous metal strip orsheet for use as a container stock.

inasmuch as a ferrous metal surface is subject to rapid corrosion whenexposed to the atmosphere, and especially in the presence of moisture orat elevated temperature, some type of an initial protective coating mustbe applied at the time of manufacture. The initially protected'surfaceshould be receptive to the usual organic coatings that are applied tosubstrates such as blackplate and tinplate, includingpaints, varnishes,lacquers and enamels, to thereby provide relatively permanent protectionagainst corrosion. The ferrous metal surface should also be receptive toorganic adhesives used in joining the side seams of containers. in viewof these requirements, a satisfactory tin free container stock shouldhave a surface with a combination of desirable characteristics,including the following:

1. The ferrous metal surface should have aninitial protective film orcoating which retards corrosion from the time of manufacture of thestrip or sheet up until it is organically coated and is ready to be usedfor preparing containers.

2. The initially protected ferrous metal surface should have a pleasingbright silver to light grey appearance, as distinguished from adiscolored or stained appearance, as often clear organic protectivecoatings are applied thereover and the substrate surface is visible.

3. The initially protected ferrous metal surface must be receptive toorganic protective coatings of the type usually applied to tinplate andblackplate.

4. The organically coated ferrous metal surface should have goodunderfilm corrosion resistance and pass the peel and sideseam adhesiontests.

5. The initially protected ferrous metal strip or sheet must also becapable of competing in price with other low cost materials as this is ahighly competitive field.

One method proposed heretofore for initially protecting the ferrousmetal substrate involves-electrodepositing thereon a substantiallynonporous layer of metallic chromium. This method produces a corrosionre-. sistant substrate surface which is also receptive to organicfinishes. However, relatively thick metallic chromium coatings arerequired of the order of l to 2 microinches or thicker, in' order toobtain sufficient corrosion protection prior to application of theorganic coating. Chromium electroplating is very inefficient as only asmall fraction of the plating current is effective in plating outmetallic chromium, and thus this method has the disadvantage ofrelatively high plating costs.

In accordance with an improved prior art process for .the manufacture oftin free container stock, a composite coating is applied to the ferrousmetal substrate which includes an extremely thin metallic chromiumundercoating and a chromium oxide-containing overcoating applied byelectrochemical treatment in a chromate bath. The undercoating ofmetallic chromium needbe only 0.l-0.5 microinch in thickness, and achromium coating of this thickness may be electrodeposited andthereafter electrochemically treated in a chromate bath to deposit thechromium oxidecontaining overcoating in a continuous electroplating.line operating at a high strip speedylt is necessary to operate-thecontinuous chromium electroplating line at a high strip speed in orderto further reduce the cost of applying the metallic chromiumundercoating. When efforts were made to increase the strip speed aboveabout 500-600 feet per minute, it was observed that the nonmetallicchromium-containing film which is normally present in small amounts onmetallic chromium plated, ferrous metal substrates increases inthickness at a rate which varies directly with increasing line speed. Itwas further observed that at strip speeds of approximately 600 feetperminute and higher, i.e., at levels of nonmetallic chromium in the filmof about 0.45-0.5 milligram per square foot per side and higher, thesurface of the ferrous metal substrate has a brownish discolorationwhich becomes more pronounced with further increases in line speed. As aresult, the appearance of the electrochemically treated chromium platedferrous metal strip is less pleasing. Heavy nonmetallic chromiumcontaining films applied during the electrodeposition of the metallicchromium coating also adversely affects the. stability-of the lubricantapplied. during oiling of the final product, luster, and ana-- lyticalresults in control tests for determining the amount of nonmetallicchromium deposited during the electrochemical treatment step. Prior tothe present invention, it was not possible to operate a chromiumelectroplating line at high speeds above about 600 feet per minute andproduce an entirely satisfactory electrochemically treated chromiumplated ferrous metal strip which is satisfactory in appearance and freeof the other disadvantages mentioned above.

It is an object of the present invention to provide a novel process forreducing discoloration of ferrous metal strip electroplated withmetallic chromium in a continuous plating line at high line speeds.

It is a further object to provide a novel process for reducingdiscoloration when electroplating ferrous metal strip with metallicchromiumin a continuous plating line, and thereafter electrochemicallytreating the chromium plated strip to deposit a chromium oxidecontainingfilm thereon.

It is still a further object to provide a novel process for reducingdiscoloration when preparing tin free ferrous metal strip useful as acontainer stock in a high speed continuous electroplating line wherein acomposite coating is applied which includes an undercoating of 0.1-0.5microinch of metallic chromium and an overcoating containing chromiumoxide.

Still other objects and advantages of the invention will be apparentupon consideration of the following detailed description and theexamples.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTSTHEREOF In accordance with the present invention, ferrous metal strip iselectroplated with chromium in a continuous chromium plating line at ahigh line speed which results in staining or discoloration of themetallic chromium plated surface. The discolored chromium plated ferrousmetal strip is intimately contacted with an aqueous solution of a watersoluble hexavalent chromium compound immediately after plating and priorto washing in fresh water or drying of the chromium electroplatingelectrolyte thereon. The plated strip is contacted with the solution inthe absence of electrolytic treatment and for a sufficient period oftime to reduce the intensity of the discoloration. Thereafter, themetallic chromium plated strip may be electrochemically treated in anaqueous electrolyte containing a water soluble hexavalent chromiumcompound to deposit thereon a' corrosion resistant film which containschromium oxide. As will be discussed in greater detail hereinafter,there are certain preferred materials, procedures and conditions whichmay be employed to produce superior results in the above steps of theinvention.

Suitable prior art chromium plating baths may be used forelectrodepositing the metallic chromium coating which is initiallydeposited on the ferrous metal strip. Prior art chromium plating bathsusually contain chromic acid, alkali metal dichromates, or other watersoluble hexavalent chromium compounds in an amount of about 50-450 gramsper liter and preferably about 100-250 grams per liter, and a catalystsuch as sulfate ion, fluoride ion, silicofluoride ion, or combinationsthereof. The catalyst may be added as a free acid producing the desiredion, such as sulfuric acid, or as a water soluble salt of the free acidsuch as the alkali metal compounds thereof. The molar ratio of thechromium' compound to the catalyst is usually between about 40:1 and200:1, and is preferably about 100:1, but any suitable prior artcatalyst concentration may be employed. One suitable electrolyte forelectrodepositing metallic chromium in high speed lines contains perliter 130-170 grams and preferably 150 grams of chromic acid, 1.0-1.4grams and preferably 1.2 grams of sulfate ion, and 1.6-5 grams andpreferably 1.8 grams of silicofluoride ion. The current density also maybe the same as is used in prior art chromium plating, and may be, forexample, about 300-2,000 amperes per square foot, and I is preferablyabout 500-l,000 amperes per square foot. The temperature of the chromiumplating bath may be in accordance with prior art practice and may beabout 100-150F., and is preferably about l-130F. A plating bathtemperature of approximately 120F. usually gives the best results.

Insoluble anodes may be used, and preferably a composite steel anodehaving a lead coating on the side facing the ferrous metal substrate tobe plated, and polyvinyl chloride or other inert insulating substanceson the opposite side.

The surface of the ferrous metal strip may be scrubbed free of oil,grease, dirt and other surface contaminants by a prior art process priorto electroplating with the metallic chromium. For example, the ferrousmetal strip may be given a wet pretreatment by electrolytically treatingin an alkaline electrolyte, washing in water, pickling in sulfuric acid,washing in water, and drymg.

While the above plating baths and conditions are satisfactory, it isunderstood that still other prior art chromium plating baths may beused. Examples of other chromium plating baths and conditions aredisclosed in United States Pat. Nos. 1,942,469, 2,177,392 and 2,415,724and in the text Modern Electroplating, edited by Fredrick A. Lowenheim,Second Edition, John Wiley & Sons, Inc., New York, New York, 1963.Chapter 5, pages -140 and the references cited on pages 128-140 areespecially pertinent as this portion of the test is directed to chromiumplating. The teachings of I this text and the cited references areincorporated herein by reference.

The ferrous metal strip is electroplated with metallic chromium in acontinuous prior art electroplating line which operates at high linespeeds. Suitable continuous high speed electroplating lines are wellknown and may be used in practicing the-present invention when employingan electrolyte in the plater as described above for theelectrodeposition of the metallic chromium coating.

The electroplating line is operated at a sufficiently high line speed tocause visible discoloration of the metallic chromium plated stripsurface due to simultaneous deposition thereon of a nonmetallicchromiumcontaining film. Usually, this occurs at line speeds of about600 feet per minute and higher, and his especially pronounced at linespeeds of about 1,000 to 2,000 feet per minute. As a general rule, aline speed which results in a nonmetallic chromium content in the filmof at least 0.5 milligram per square foot per side of strip surface willgive discoloration, and when the nonmetallic chromium content reachesabout 0.6-0.7 milligram per square foot per side and higher,- thediscoloration is very noticable. 1n instances where the nometallicchromium content in the film is about 0.9-1.0 milligram per square footper side and higher, which usually occurs at line speeds of 1,000-2,000feet per minute, then the product is often considered to beunsatisfactory for use as a container stock in many commercialapplications.

The thickness of the metallic chromium coating may be in accordance withprior art practice. Usually metallic chromium coatings of 10 microinchesand less are electrodeposited at high strip speeds which give staining,and preferably less than 4 microinches. Most container stocks havemetallic chromium coatings of l-3 microinches or less.

The invention is especially useful in preparing ferrous metal containerstock which has a composite coating including an extremely thinundercoating of metallic chromium, such as 0.1-0.5 microinch.Unsatisfactory product performance is achieved due to insufficientmetallic chromium at coating thicknesses less than about 0.1 microinch,and metallic chromium coatings above 0.5 microinch are more expensive toapply and add appreciably to the cost of manufacturing tin free steelfor container stock due to the inefficiency of the chromium platingprocess. The best results are usually achieved by maintaining themetallic chromium coating at a thickness of about 0.1-0.3 microinch. Ametallic chromium coating having a thickness of about 0.1 microinch issatisfactory for container stock to be used in the preparation ofcontainers for the storage and preservation of beer and the lesscorrosive beverages and food stuffs, whereas metallic chromium coatingsof 0.3 microinch and above give better results when storing andpreserving the more corrosive food stuffs and beverages.

Thin metallic chromium coatings having thicknesses of 0.1-0.5 microinchdo not provide satisfactory initial protection against corrosion of theferrous metal substrate in the absence of further treatment. As aresult, it is necessary to electrochemically treat the thin coatings torender them satisfactory for container stock and thereby increase thecorrosion resistance prior to application of a permanent coating such asan organic coating. The corrosion resistance of heavier chromiumcoatings also may be improved by electrochemical treatment or otherdesirable properties may be imparted thereto. However, the discolorationon the metallic chromium coated strip surface must be reduced to anacceptable level prior to the electrochemical treatment as otherwise anunsatisfactory final product will be produced. The term discolored filmor stain as used herein is intended to embrace all undesirable filmsdeposited in the chromium electroplating zone simultaneously with themetallic chromium coating.

It has been discovered that the discoloration'or stain on the metallicchromium plated ferrous metal strip may be reduced markedly byintimately contacting it with an aqueous solution containing a watersoluble hexavalent chromium compound. Preferably the solution' iscontacted with the discolored or'stained strip at a level ofelectrolytic treatment below the threshold of metallic chromiumdeposition. The chromium compound may be chromic acid, alkali metalchromates and dichromates, or other suitable water soluble hexavalentchromium compounds. The solution preferably contains the hexavalentchromium compound ina concentration which approximates the rangesgivenherein for the chromium plating electrolyte, and has a pH value ofabout 2 or less and about 0.5-1 for better results. In one imporatantvariant, the solution has substantially the same ingredients and/orconcentrations of ingredients as exist in the chromium platingelectrolyte. In another important variant, the metallic chromium platedstrip is treated in the chromium electroplating electrolyte subsequentto the metallic chromium electrodeposition step. In this latter variant,the electrolyte is preferably contacted with the discolored plated stripin the absence of electrolytic treatment and prior to withdrawing theplated strip from the chromium plating electrolyte.

The temperature of the solutionof hexavalent chromium compound to becontacted with the. strip may have approximately the same temperature asthe electrolyte, but higher temperatures up to the boiling point may beused to reduce the period of treatment. For instance, the temperaturemay be about l00-l90F.,

' and often is preferably about l00-l30l-".

The aqueous solution of the hexavalent chromium compound is contactedwith the discolored strip for a sufficient period of time to reduce theintensity of the discoloration and to lower the nonmetallic chromiumcontent in the film deposited simultaneously with the metallic chromiumto a desirable level. This may vary reduced to approximately that whichnormally eiiists' at plating speeds below 500. feet per minute, such as0.3-0.4 milligram or less per square foot per side.

The period of treatment necessary to achieve the above mentioned levelsof nonmetallic chromium in the film will vary somewhat. However,treatment for at least 0.3 second, and preferably for at least 0.5second usually gives satisfactory results. The upper limit on the periodof treatment is practical in nature as the solution does not'attack themetallic chromium coating and treatment periods of several minutes induration do not adversely affect the product. However, treatment forlonger than about 1 or 2 seconds is usually not preferred in high speedplating lines. I

The discolored strip is treated with the solution of the hexavalentchromium compound immediately after it has been electroplated with themetallic chromium, and prior to washing with fresh water or drying of afilm of the metallic chromium plating electrolyte thereon; Otherwise,satisfactory results usually are not achieved. Agitation is notnecessary, but may be used if desired. Satisfactory'results are obtainedbypassing the discolored strip through a bath of the treating solutionat the existing line speed. The rapidly moving strip seems to inducesufficient agitati'onin the vicinity of the strip surface to cause thenonmetallic chromium film to be somewhat from product to product, but asa general I rapidly dissolved. v

After reducing the discoloration of the strip, it may be given anelectrochemical treatment in an aqueous solution of a water solublehexavalent chromium compound such as chromic acid, alkali metaldichromates including sodium or potassium dichromate, and other watersoluble hexavalent chromium-containing compounds. Chromic acid isusually preferred, and satisfactory baths may contain about 20-50 gramsper liter and preferably about 30 grams per liter of chromic acid, orabout 0.5-2 ounces, and preferably about l ounce per gallon of sodium orpotassium dichromate. If desired the bath may alsocontain a small amountof sulfate ion (80,"), fluoride ion (F'), silicofluoride ion (SiF,' andmixtures thereof, such as up to about 0.5 grams per liter and preferablyabout 0.1-0.4 grams per liter. One very satisfactory electrochemicaltreatment bath contains per liter 30-40 grams and preferably about 35grams of chromic acid, 0.05-0.2 gram and preferably about 0.1 gram ofsulfate ion (80,"), and 0.3-0.6 gram and preferably 0.4 gram ofsilicofluoride ion (SiF- The electrochemical treatment bath may beoperated often ata rate of about 200 amperes per square foot for- 0.5second to thereby provide about coulombs per square foot of current,using a bath temperature of about F.

As a general r ule, about 0.6-5.0 milligrams per square foot per side oftotal chromium should be present in the film that is deposited in'theelectrochemical treatment step. The cathodic electrochemical treatmentresults in the deposition of a chromium oxidecontaining film which alsocontains metallic chromium in instances where a small amount of achromium plating catalyst is present. The chromium content of thechromium oxide seems to be largely in the plus 3 valence state, and mayinclude hydrated Cr,O Preferably, the film should contain about 1.0-1.5milligrams of nonmetallic chromiumper square foot per side in thechromium oxide content of the film.

The electrochemical treatment is especially useful as a post treatmentfor metallic chromium coatings having a thickness of about 0.1-0.5microinch as it markedly improves the corrosion resistance. It isthought that for some unexplained reason the metallic chromium contentof the electrochemically deposited film tends to seek out the pores inthe initially deposited metallic chromium film, and thereby renders itless porous. In addition to this, the chromium oxide content of theelectrochemically deposited film tends to cover and further protect theinitial metallic chromium coating. A surface film or coating is alsoproduced on the ferrous metal strip which renders the surface highlyreceptive to prior art organic coatings.

Organic coatings of the type, usually applied to tin free steelcontainer stock or tinplate to further improve the corrosion resistanceand to provide relatively permanent protection against corrosion may beapplied over the electrochemically treated surface. While the inventionis not limited thereto, examples of suitable organic coatings includingphenolic, modified phenolic, epoxy, modified epoxy, vinyl resin, teflon,and drying oil based paints, varnishes, lacquers and enamels. Theelectrochemically treated surface also forms an excellent base for sideseam adhesives which are sometimes used in the manufacture ofcontainers.

The present invention is especially useful for produc- EXAMPLE Coils of55 pound double reduced blackplate are cleaned by a prior art procedureto remove grease, dirt and other foreign matter from the surface. Thetreatment includes cathodic and anodic electrolytic treatment in acaustic cleaner (Orthosil), a rinsing with water, pickling in dilute(2-3percent) aqueous sulfuric acid, scrubbing and rinsing with water anddrying. The clean strip is then passed continuously through a series ofvessels in a prior art high speed electroplating line wherein it iselectroplated with metallic chromium, passed through a body of asolution of a hexavalent chromium compound to reduce discoloration,electrochemically treated to deposit a chromium oxidecontaining film,rinsed with water, dried, oiled and coiled.

The electrolyte for the chromium plating bath contains 150 grams perliter of chromic acid, 1.2 grams per liter of sodium sulfate calculatedas sulfate ion (804), and 1.8 grams per liter of sodium silicofluoridecalculated as silicofluoride ion (SiFJ). The temperature of theelectrolyte is maintained at 120F., and the current density at about800-l,000 amperes per square foot. The anodes in the chromium plater arelead coated steel on the surfaces facing the strip, and are coated withpolyvinyl chloride on the opposite side thereof. The amount of metallicchromium deposited on the strip is 0.3 microinch.

The metallic chromium plated strip is passed through a body of asolution of a hexavalent chromium compound immediately after completingthe electroplating step, and intimately contacted therewith for a periodof 1 second. The solution contains the same ingredients andconcentrations of ingredients as the chromium plating electrolyte toavoid introducing other substances into the system, and the temperatureis 120F. The path of the strip through the body of solution islengthened or shortened as necessary to provide the one second treatmentperiod when making runs at varying speeds.

The metallic chromium plated strip is withdrawn from the solution of thehexavalent chromium compound and is passed into an electrochemicaltreating vessel filled with an electrolyte containing 35 grams per literof chromic acid, 0.1 gram per liter of sodium sulfate calculated assulfate ion ($0 and 0.43 gram per liter of sodium silicofluoridecalculated as silicofluoride ion (SiFf). The electrolyte temperature ismaintained at 120F., and the strip surface is treated cathodicallyfor'0.4 second at 250 amperes per square foot to thereby provide asurface treatment of coulombs per square foot. The strip is passedbetween steel anodes coated with lead on the surface facing the strip.This treatment results in about 1.4 milligrams per square foot ofchromium oxide in the film deposited on the strip. i

The electrochemically treated strip is withdrawn from theelectrochemical treating vessel, rinsed in water, dried, oiled andcoiled. Samples of strip are cut from the various runs and tested.

Upon making runs at strip speeds of 300, 600, 800, 1,000 and 1,300 feetper minute and observing the samples, no discoloration is noted. Thesample for the run made at 1,300 feet per minute is as lustrous and asfree from discoloration as the sample from the run made at 300 feet perminute.

EXAMPLE ll The general procedure of Example I is repeated with theexception of omitting the 1 second treatment of thechromium plated stripin the body of the solution of the hexavalent chromium compound. Thesamples are observed for discoloration as in Example I.

The sample from the run made at 300 feet per minute is lustrous and freeof discoloration. However, the samples from the remaining runs arediscolored with the intensity of the discoloration varying directly withthe strip speed.

EXAMPLE III The general procedure of Example I is repeated with theexception of omitting the 1 second treatment of the stained metallicchromium plated strip in the solution of the hexavalent chromiumcompound, and subsituting therefore a one second treatment in thechromium electroplating electrolyte. The stained strip was contactedwith the plating electrolyte subsequent to the chromiumeletrodepo'sition step and in the absence of electrolytic treatment. Theresults obtained in this Example are comparable with the resultsreported above for Example I.

I claim:

1. In a process for preparing electrochemically treated chromium platedferrous metal strip in which ferrous metal strip is passed through acontinous elecgrams per square foot per side of chromium, the aqueouselectrolyte for said electrochemical treatment con-.

- taining about 30-35 g/ l of chromic acid and a catalytic fluoride ionand silicofluoride ion, the molar ratio of electroplated strip iselectrochemically treated as a cathode in an aqueous electrolytecontaining about 30-35 g/ l of chromic acid to deposit thereon anonmetallic chromium-containing film including chromium oxide, thesurface of the resulting electrochemically treated metallic chromiumelectroplated strip having a composite nonmetallic chromium-containingfilm thereon containing a total of about 0.6-5 milligrams per squarefoot per side of chromium and being discolored, the improvement incombination therewith comprising reducing the discoloration of theelectrochemically treated chromium plated ferrous metal strip byintimately contacting the surface of the metallic chromium electroplatedstripwith an aqueous solution of chroinic acid, said aqueous solution ofchromic acid having substantially the same ingredients and about thesame concentrations of ingredients as the chromium electroplatingelectrolyte, the metallic chromium electroplated strip being submersedin said aqueous solution of chromic acid and being intimately contactedtherewith in, the absence of substantial electrolytic treatment untilthe nonmetallic chromium content in said initial nonmetallicchromium-containing film is reduced to not more than 0.4 milligram persquare foot per side, the metallic chromium electroplated strip beingintimately contacted with said aqueous solution of chromic acid afterelectroplating said coating of metallic chromium thereon and prior tosaid electrochemical treatment, the metallic chromium electroplatedstrip being intimately contacted with said aqueous'solution .of chromicacid without drying said chromium electroplating electrolyte thereon andbefore contacting the metallic chromium electroplated strip with amountof a catalyst selected from the group consisting of sulfate ion andadmixtures thereof with at least one additional substance selected fromthe group consisting of fluoride ion and silicofluoride ion.

2. The process of claim 1 wherein the ferrous metal strip iselectroplated with a coating of metallic chromium having a thicknessbetween about 0.1 microinch and- 0.5 microinch, the said aqueoussolution of chromic acid is chromium electroplating electrolyte and themetallic chromium electroplated strip is contacted therewith over aperiod of at least 0.3 second, the nonmetallic chromium content in saidinitial nonmetallic chromium-containing film is reduced to about 0.3-0.4milligrams per square foot per side, and said composite nonmetallicchromium-containing film' contains not more than 1.5 milligrams ofchromium per square foot per side.

3. The process of claim 2 wherein the metallic chromium electroplatedstrip isintimately contacted with the said aqueous solution of chromicacid over a period between=about 0.3 second and 2 seconds.

4.'Theprocess of claim '3 wherein the strip speed is at least 1,000 feetper minute, said initial nonmetallic chromium-containing film containsat least 0.6 milligram'of chromium per square foot per side, and saidcomposite nonmetallic chromium-containing film contains about 1.0-1.5milligrams of chromium per square foot per side. H

5. The process of claim 1: wherein said chromium electroplatingelectrolyte contains about l ;g/l of chromic acid. e

6. The process of claim 1 wherein the electrolyte for saidelectrochemical treatment contains about 30 g/l of chromic acid. r

7. The process of claim 1 wherein said chromiumelectroplatingelectrolyte contains about l50 g/l of chromic acid and theelectrolyte for said electrochemical treatment contains about 30 g/l ofchromic acid.

8. The process of claim 1 wherein the ferrous metal strip iselectroplated with a coating of metallic chromium having a thicknessbetween about-0.1 microinch mium electroplated strip is intimatelycontacted with said aqueous solution of chromic acid until thenonmetallic chromium content of said initial nonmetallicchromium-containing film is reduced to about 0.3-0.4 milligram persquare foot per side. I

'11. The process of claim 1 wherein said aqueous solution of chromicacid is chromium electroplating electrolyte.

- 'aar: a a

2. The process of claim 1 wherein the ferrous metal strip iselectroplated with a coating of metallic chromium having a thicknessbetween about 0.1 microinch and 0.5 microinch, the said aqueous solutionof chromic acid is chromium electroplating electrolyte and the metallicchromium electroplated strip is contacted therewith over a period of atleast 0.3 second, the nonmetallic chromium content in said initialnonmetallic chromium-containing film is reduced to about 0.3-0.4milligrams per square foot per side, and said composite nonmetallicchromium-containing film contains not more than 1.5 milligrams ofchromium per square foot per side.
 3. The process of claim 2 wherein themetallic chromium electroplated strip is intimately contacted with thesaid aqueous solution of chromic acid over a period between about 0.3second and 2 seconds.
 4. The process of claim 3 wherein the strip speedis at least 1, 000 feet per minute, said initial nonmetallicchromium-containing film contains at least 0.6 milligram of chromium persquare foot per side, and said composite nonmetallic chromium-containingfilm contains about 1.0-1.5 milligrams of chromium per square foot perside.
 5. The process of claim 1 wherein said chromium electroplatingelectrolyte contains about 150 g/1 of chromic acid.
 6. The process ofclaim 1 wherein the electrolyte for said electrochemical treatmentcontains about 30 g/1 of chromic acid.
 7. The process of claim 1 whereinsaid chromium electroplating electrolyte contains about 150 g/1 ofchromic acid and the electrolyte for said electrochemical treatmentcontains about 30 g/1 of chromic acid.
 8. The process of claim 1 whereinthe ferrous metal strip is electroplated with a coating of metallicchromium having a thickness between about 0.1 microinch and 0.5microinch.
 9. The process of claim 1 wherein the ferrous metal strip iselectroplated with a coating of metallic chromium having a thicknessbetween about 0.1 microinch and 0.5 microinch, the strip speed is atleast 1,000 feet per minute and said initial nonmetallicchromium-containing film contains at least 0.6 mIlligram of chromium persquare foot per side.
 10. The process of claim 1 wherein the metallicchromium electroplated strip is intimately contacted with said aqueoussolution of chromic acid until the nonmetallic chromium content of saidinitial nonmetallic chromium-containing film is reduced to about 0.3-0.4milligram per square foot per side.
 11. The process of claim 1 whereinsaid aqueous solution of chromic acid is chromium electroplatingelectrolyte.